Process for manufacturing an article in the form of a slab and article manufactured

ABSTRACT

A manufacturing process produces an article in slab form and more precisely an article made from a hydrated matrix which contains cement, sand, water and reinforcing fibers. The process includes the steps of mixing the constituents until a substantially homogeneous mineral paste is obtained; extruding the mineral paste in the form of a sheet having a thickness approximately equal to that of the article; and cutting the sheet to the size of the article. The article is made in the form of a slab and may be an imitative slate piece to be used as a roofing cover element.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The invention relates to a process for manufacturing an article in slabform and more precisely to an article comprising a hydrated matrix whichcontains cement and sand and is reinforced by fibres.

2. Description of the Related Art

The invention will be more particularly described for the manufacture ofbuilding covering elements, such as building roofing slabs imitatingslate. However, the invention is not limited to this type of article andshould be interpreted in a non-limiting way and as encompassing any typeof article comprising a hydrated matrix which contains cement and sandand is reinforced by fibres.

It has already been proposed in European Patent Application Serial No.165,388 to produce building roofing slabs imitating slate. It turns outthat the articles obtained in accordance with the teaching of thisdocument have a relatively high relative density (greater than 2) andare brittle. This brittleness prevents them from being produced with asmall thickness without the risk of fracturing or cracking. The articlesthus obtained are therefore unsuitable for forming roofing slabs.

The subject of the invention is thus a manufacturing process resultingin the production of thin slabs which may be conveniently used forbuilding coverings, especially as roofing, and have the appearance ofslate.

The French Patent Application Serial No. 93/15184 of the Applicant hasalready proposed such a manufacturing process. The process describedaccording to this document includes in particular a rolling step whichhas two functions. First of all, this step confers the desired thicknesson the articles produced. The second function of the rolling is toimprove the orientation of the reinforcing fibres with the aim ofobtaining isotropic mechanical properties. Although the process proposedby this document leads to lighter articles with the desired thicknesswithout thereby increasing their brittleness, it does, however, requireworking conditions which are not very satisfactory because the rolling,which is carried out by contact with a succession of rolls, may run therisk of the matrix sticking on the rolls. Apart from the risk ofdegrading the matrix and therefore of increasing the production costsbecause of the loss of material, it is necessary to clean the equipmentvery frequently. This maintenance also leads to an increase in thecosts. Should the matrix stick on the rolls, the latter may in factcarry off material which hardens and modifies the appearance of thesurface of the rolls. The rolling operations are then renderedineffective. It is therefore necessary to provide frequent cleaning ofthe equipment, which cleaning decreases the production rates andincreases the cost.

SUMMARY OF THE INVENTION

The object of the invention is to provide a method for making thin slabsfor building coverings. The method does not have the previouslymentioned drawbacks, especially the fouling of the equipment during therolling step and the risk of degrading the product being produced duringthis same step.

This object is achieved according to the invention by a process formanufacturing an article in the form of a slab, comprising a hydratedmatrix which contains especially cement and sand and is reinforced byfibres, the said process comprising the following steps:

a) the constituents are mixed until a substantially homogeneous paste isobtained;

b) the paste is extruded in the form of a sheet having a thicknessapproximately equal to that of the article; and

c) the sheet is cut to the size of the article.

The process thus described makes it possible to manufacture articles inslab form, especially those intended for building roofing slabs andtherefore having, for the desired thickness, mechanical properties and adensity which are suitable. The process according to the invention hasmeant, in particular, omitting the rolling step, with which the variousdrawbacks mentioned above were associated.

Although it is clear that the first function of the rolling step of theprocess mentioned previously is, according to the invention, fulfilledby the step consisting of extruding the paste in the form of a sheethaving a thickness substantially equal to that of the article, it is notobvious that a result similar to that corresponding to the secondfunction may be obtained. This is because the first function, whichconsists in conferring the desired thickness on the articlesmanufactured, is clearly replaced according to the invention byextruding the paste directly to the desired size of the finishedarticle. As regards the second function of the rolling step, consisting,according to the process of the prior art, in reorient ing thereinforcing fibres in the already extruded paste, it appears,unexpectedly, that the reorientation of the fibres is not necessarybecause of the extrusion as proposed. It seems that an extrusion stepwhich ends in a paste thickness close to or equal to that desired forthe finished article, as the invention teaches, results in atwo-dimensional orientation of the fibres in the paste, this orientationbeing satisfactory for the applications envisaged.

According to a preferred mode of the invention, provision is made tofilter the paste before extrusion in order to remove impurities having adimension similar to the thickness of the slab. It is not uncommon infact for impurities having a dimension similar to or greater than thethickness of the article to form; these may be, for example, aggregatesof raw materials or strands of reinforcing fibres. This filtrationprevents fouling of the extrusion head which could disrupt themanufacture of the articles. It can be carried out, for example, by anadvantageously mounted screen pack which is accessible so as to beeasily cleaned and positioned sufficiently upstream of the extruder headso as not to disturb the manufacture when it becomes fouled andpartially obstructed. This filtration furthermore improves the qualityof the products, especially compared to the process mentioned previouslywhich includes a rolling step. This is because in that process, whensuch impurities appear, they are ground up during the rolling step;however, this creates a defect in the finished article which is thenscrapped. According to the invention, this type of defect no longerappears and articles of uniform quality are therefore obtained.

In a variant of the invention, provision is made to shape the articleinto the form of a slab by a vertical pressing operation. During thisstep, it is possible, for example, to confer on the upper face of thearticle the appearance of a natural slate by means of an upper mould.

A vertical pressing operation enables the article to be shaped withoutrisk of disturbing its thickness. This is because, in the techniqueproposed in French Patent Application Serial No. 93/15184, the pressinitially makes an angle with the sheet to be shaped, which may disturbthe uniformity of the article, since the material may creep at thisstage in the production.

In another variant of the invention, in which the paste is extruded witha thickness slightly greater than that of the article, the verticalpressing operation is carried out between an upper mould and a lowercounter-mould which include nonplanar shapes.

This other variant makes it possible, for example, to produce coveringelements which include reliefs serving to fix the elements to a supportand/or to fix them together.

The articles produced according to one or another of these variants maytherefore have one face imitating slate and possibly include shapes tobe used especially for fixing and for sealing.

With a view to further improving the surface finish of the articleimitating slate, in such a way that it resembles even more a naturalslate, the invention makes provision for accompanying the verticalpressing operation with vibration. This vibration especially reduces theporosity of the surface by closing up the sink marks. This vibration isadvantageously a vertical vibration whose frequency is between 30 Hz and150 Hz and preferably between 75 and 100 Hz and more preferably equal to87 Hz and whose amplitude is less than 2 mm and preferably equal to 0.9mm.

As regards the pressure, this is between 0.5 and 5 bar and in particularcan be lower when the pressing operation is accompanied by vibration.

The invention also provides an article in the form of a slab, obtainedaccording to the process which has just been described. This article inthe form of a slab is characterized by a hydrated matrix which containscement and sand and is reinforced by fibres, the hydrated matrixcontaining 100 parts by weight of cement, 80 to 120 parts by weight ofsand with a particle size of less than 1 mm and the fibres beingalkali-resistant glass fibres present in a proportion of between 3 and4% by weight with respect to the dry matter without the glass, thearticle preferably having a relative density not exceeding 1.85.

A glass-fibre content of less than 3% leads to insufficientreinforcement. A content greater than 4% may lead to difficulties of aTheological nature, that is to say the paste exhibits insufficientplasticity, especially for its shaping treatment. In a preferredvariant, the fibre content is less than 3.5% in order to guaranteesatisfactory plasticity and to improve the production results.

The glass fibres preferably have a length/diameter ratio of betweenapproximately 100 and 200:1. The composition of the article thusdescribed is especially well suited to the manufacturing processdescribed previously. It turns out in particular that the glass-fibrecontent is less than that recommended by French Patent ApplicationSerial No. 93/15184, but leads to perfectly satisfactory results, thearticles obtained having good mechanical properties for the desiredthickness and, in particular, good ductility.

According to an advantageous form of the article, the latter has asurface with a porosity whose pores are of a very small diameter, saidsurface thus appearing locally smooth. This smooth appearance is onlylocal, especially in the case of a surface having the appearance of aslate.

The water content is preferably between 16.5 and 21.5% by weight withrespect to the dry matter. Below 16.5% it is no longer possible toprocess the paste and above 21.5% free water remains in the paste,possibly leading to excessive porosity in the finished product.

The hydrated matrix may advantageously contain ultrafine particles,especially silica fume or metakaolin, in a quantity of 8 to 18 parts byweight and preferably equal to 10 parts by weight. The function of theseparticles is especially to facilitate the extrusion by improving theplasticity of the paste and to improve the mechanical properties of thearticle. These effects are insufficient below 8 parts by weight, whileabove 18 parts by weight the plasticity drops excessively duringextrusion. The proposed metakaolin is kaolin treated at 800° C. so thatit becomes pozzolanic. The expression "ultrafine particles" is used tomean, in general, particles whose BET specific surface area is between10 and 30 m² /g.

Still in order to improve the extrusion, and especially to improve theslip effect of the paste, the invention advantageously provides for theaddition of an extrusion agent chosen from the cellulose ethers, such asmethylhydroxyethylcellulose (MHEC), in a quantity of approximately 1part by weight.

The matrix may also contain, for the purpose of coloring the article, acolorant, such as carbon black, in a quantity of 0.5 to 2 parts byweight and preferably of 1 part by weight. Choosing carbon blackadvantageously leads to a color which is very similar to that of naturalslate and is stable over time.

In a preferred variant of the invention, the article has at least oneface imitating slate.

In this case, it preferably has a thickness in its central part of lessthan 1 cm and preferably less than 6 mm.

Such an article then has all the properties required for forming abuilding covering element having good mechanical properties.

Such articles may therefore be use as building roofing elements in thesame way as natural slates.

In another variant, the article, along one edge, has catching reliefsprojecting downwards and an upper surface which imitates slate and ispossibly provided with ribs. The concepts of reliefs projectingdownwards and an upper imitative surface are related to the position ofthe article used as a roofing element. According to this variant, thearticles allow, especially when fitting them, small overlap regions,corresponding, that is to say, to the region of the upper ribs.

BRIEF DESCRIPTION OF THE DRAWINGS

Further details and advantageous characteristics of the invention willemerge from the description of the embodiments according to theinvention, given with reference to FIGS. 1, 2, 3, 4 and 5, which depict:

FIG. 1 a diagram of a perspective view of an article in accordance withthe invention;

FIG. 2 a diagram of a perspective view of another article according tothe invention;

FIG. 3 a diagram of a device for implementing the invention;

FIG. 4 a diagram of one section in the device for producing an article,as depicted in FIG. 1; and

FIG. 5 a diagram of the same section in the device for producing anarticle as depicted in FIG. 2.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The various figures are diagrams of examples of products orinstallations according to the invention, the sizes of which are notdrawn to scale. They are simplified and certain details are drawn out ofproportion so as to make the invention simpler to understand.

FIG. 1 depicts a first possible embodiment according to the invention.This is a "flat" thin slab 1 imitating slate. Such slabs 1 are producedwith a black or anthracite color and have a face 5 which includes arelief, shown diagrammatically in part in the figures, imitating thesurface of a natural slate. They have a thickness in their central partof about 5 to 6 mm.

According to another embodiment of the invention, depicted in FIG. 2,the slab 1 has a thickness in its central part of about 9 mm and isprovided along one edge 2, forming in service the upper edge of theslab, with catching reliefs 3 projecting downwards with a height ofabout 8 mm; it also has joint-ribs 4 on its upper surface 5, that is tosay on the surface imitating the surface of a natural slate, along theupper edge and along the two side edges. The side edges have a thicknesswhich is reduced by approximately a half, one from the top and the otherfrom the bottom, and their ribs 4 are, in the first case, directedupwards and, in the second case, downwards, so as to allow a slab 1 tofit into the adjacent slabs. In addition, the lower edge of the slab 1has a recess, not depicted in the figure, designed to receive the rib 4of the upper edge of the slabs 1 lying immediately above it.

The slabs 1 typically have dimensions ranging from 200×300 mm to 300×600mm and possibly 600×600 mm, and are relatively light, with a relativedensity of about 1.8.

In each embodiment, the upper surface of the slab 1 has irregularitiesin the face 5 imitating slate, called "slating cleavage", showndiagrammatically in part in the figures.

The slabs 1 are ductile: they have a tensile strength of about 14 to 18MPa and an elongation at breakage of at least 0.15%.

These slabs 1 may therefore be used for the roofing of a building. Thoseslabs 1 in FIG. 1 will be installed conventionally, like natural slates,and those depicted in FIG. 2 allow interlocking by means of the ribs 4which limit the overlaps of the slabs 1 one with another.

FIG. 3 illustrates highly diagrammatically a device for producing theslabs 1. First of all, a paste is prepared by mixing a number ofconstituents described hereinbelow. The mixing, not depicted in thefigures, may be carried out by any technique known to one skilled in theart, for example, in a vessel, or in one or more mixing screws which mayalso be used for the extrusion.

The starting paste is produced with the following constituents, whichform a hydrated matrix reinforced by glass fibres:

1. Matrix

1.1. Dry matter

High-quality Portland cement, for example HPR cement: 100 parts byweight,

Silica fume: 10 parts by weight,

Fine sand of particle size less than 800 μm: 100 parts by weight,

Extrusion agent: methylhydroxyethylcellulose (MHEC), approximately 1part by weight,

Black colorant (carbon black): approximately 1 part by weight;

1.2. Water: 19% by weight with respect to dry matter.

2. Reinforcing fibres

These are alkali-resistant glass fibres present in a proportion of 3.5%by weight with respect to the dry matter without the glass. These fibresmay especially be fibres having a length of 6 to 12 mm, consisting oftwo hundred 14 μm diameter filaments, i.e. a length/diameter ratio ofbetween approximately 100 and 200:1. Suitable glass fibres are fibreswith a water-insoluble size, these being available commercially underthe name "Cemfil".

All the constituents described above are mixed relatively intensely andbriefly in order to obtain a thick paste, roughly having the consistencyof a plasticine, in which the glass fibres have remained intact.

After the mixing step, the paste is conducted along an extruder screw 6and then extruded in the form of a sheet 7 with a thicknessapproximately equal to that of the slab 1 in the finished state. Thedimensions of the sheet 7 are obtained by the choice of a suitableextruder head or nozzle. The strip 7 is taken up on a conveyor belt 8.Placed within the device or extruder screw 6 is a filter, not depictedin the figures, which will remove all the impurities which could disruptthe extrusion. As was stated earlier, these impurities may form fromaggregates of raw materials or from strands of fibres and may reach thedesired size of the article and, consequently, of the extruder head. Inorder to prevent the latter from becoming obstructed and therefore fromdisrupting the formation of the sheet 7, it is preferable to stop theseimpurities before they have reached the extruder head. The filter mayconsist of a screen pack, the slits of which are chosen so as to retainthe impurities which may disrupt the manufacture. The screen pack isadvantageously mounted sufficiently upstream of the extruder head sothat, when the former becomes fouled by the presence of impurities, theextrusion cannot be disrupted. Moreover, the screen pack is designed tobe accessible, in order to be cleaned easily, frequently and withoutinterrupting the production.

A transverse-cutting device 9 forms, from the sheet 7, a succession ofblocks 10 which then have approximately the dimensions of the finishedslab 1. These blocks 10 are then taken up by a second conveyor 11 whichguides these blocks 10 to a third conveyor 12 carrying a succession ofsupports 13. This third conveyor 12 goes under the conveyor 11 and makesan angle with the latter so as to create a point where these twoconveyors meet. The speeds are synchronized in such a way that, at theend of the conveyor 11, each support 13 receives one block 10, always inthe same relative position.

The supports 13 will be described in more detail later, especially withreference to FIGS. 4 and 5. Each block 10 carried by a support 13 isthus conducted to a shaping unit.

This shaping unit includes one or more shaping plates 14 which fit overand press down on one or more blocks 10 simultaneously. A single plate14 is depicted in FIG. 3. The lower surface of the plate 14 reproducesthe pattern or relief of the surface of natural slate; this pattern willpreferably have been moulded directly from a natural slate. The pressingtime is generally less than 10 seconds and preferably about 5 seconds.The pressure exerted by the plate 14 during shaping is between 0.5 and 5bar. This pressure may be decreased especially when it is accompanied byvibration. The function of this vibration is essentially to improve thesurface finish of the upper face of the slab 1, that is to say the faceimitating the appearance of slate. The use of this vibration makes itpossible to obtain, locally, a smoother surface while still preservingthe imitation slate relief; it seems that the vibration makes itpossible to close up the pores or the "micro-defects" appearing at thesurface of the blocks 10. The use of vibration also contributes toimproving the Theological properties of the material.

In the case of the production of a slab 1 in accordance with FIG. 1 andhaving dimensions of 300×600 mm, the vibration is determined by thefollowing parameters: a frequency of 87 Hz and an amplitude of 0.9 mm;the pressure exerted by the shaping plate 1 is then approximately 1 bar.

The vibration described above may be produced by any means known to oneskilled in the art, for example by one or more vibrators 17 connected tothe plate 14 and transmitting the vibration via the plate 14.

The vibration mentioned above also has the advantage of simplifying thedemoulding or removal from the shaping plate 14 after pressing a block10, as the material of which the block 10 is composed does not stick atall and detaches very simply from the plate 14.

FIG. 4 diagrammatically illustrates the final phase of pressing a blockwith a view to obtaining a slab 1 in accordance with that depicted inFIG. 1. Likewise, FIG. 5 depicts this same phase for a slab 1 inaccordance with that depicted in FIG. 2. The productions of these twotypes of slab 1 differ in a number of ways, especially in the choice ofdifferent shaping plates 14 and supports 13, which may be termedrespectively the upper mould 14 and the lower counter-mould 13.

In the case of the flat slabs 1 in FIG. 1, the shaping takes place onflat supports 13 whereas in the case of the slabs 1 having reliefs 3 and4, in FIG. 2, supports 13 are used which have recesses 15 into which thepaste is forced during shaping in order to form the reliefs 3. Asregards the upper reliefs 4, these are formed by recesses 16 with whichthe shaping plates 14 are provided.

Moreover, in the case of the production of slab 1 according to FIG. 1,the extruded sheet 7 has dimensions similar to the finished article butslightly greater, so that the amount of material making up one block 10is sufficient to form the reliefs or ribs 3 and 4 and therefore to fillthe recesses 15 and 16.

Moreover, the lower recesses 16 of the shaping plates 14 may be producedfrom various imprints of natural slates, each imprint advantageouslycorresponding to the same volume. In this manner, since the amount ofmaterial of each block 10 is the same, it is possible to avoid as far aspossible any risk of material flash during shaping.

Following the pressing step, the supports 13 carrying the slabs 1 havingthe desired shape are conducted with the slabs 1 to the final stage ofmanufacture which consists in stoving them and varnishing them in orderto obtain the final, slate-imitating, relatively light and ductileproduct which was described previously.

We claim:
 1. Process for manufacturing an article in the form of a slabhaving a hydrated matrix which contains especially cement and sand andis reinforced by fibres, characterized in that the process comprises thefollowing steps:a) mixing the constituents of the matrix, i.e., cement,sand, water and reinforcing fibres until a substantially homogeneousmineral paste is obtained; b) extruding the mineral paste in the form ofa continuous sheet having a thickness approximately equal to that of thearticle; and c) cutting the sheet to the size of the article.
 2. Processaccording to claim 1, characterized in that the process furthercomprises, after the mixing step but before the extruding step, a stepof filtering the mineral paste in order to remove large impuritieshaving a dimension similar to the thickness of the slab.
 3. Processaccording to claim 1, characterized in that the process furthercomprises, after the cutting step, a step of shaping the article intothe form of a slab by a vertical pressing operation.
 4. Processaccording to claim 3, in which the mineral paste is extruded in the formof a sheet having a thickness approximately equal to but greater thanthat of the article, characterized in that the shaping step furthercomprises a substep of carrying out the vertical pressing operationbetween an upper mould and a lower counter-mould which include nonplanarshapes.
 5. Process according to claim 3, characterized in that thevertical pressing operation is accompanied by vibration.
 6. Processaccording to claim 5, characterized in that the vibration is verticaland in that its frequency is between 30 Hz and 150 Hz and preferablybetween 75 Hz and 100 Hz and more preferably equal to 87 Hz, and in thatits amplitude is less than 2 mm and preferably equal to 0.9 mm. 7.Process according to claim 3, characterized in that during the pressingoperation the pressure is between 0.5 and 5 bar.
 8. Process according toclaim 2, characterized in that the process further comprises, after thecutting step, a step of shaping the article into the form of a slab by avertical pressing operation.
 9. Article in the form of a slab comprisinga hydrated matrix which contains especially cement and sand and isreinforced by fibres, characterized in that the hydrated matrix contains100 parts of weight of cement and 80 to 120 parts by weight of sand witha particle size of less than 1 mm and in that the fibres arealkali-resistant glass fibres present in a proportion of between 3 and4% by weight with respect to the dry matter without the glass. 10.Article according to claim 9, characterized in that the article has arelative density not exceeding 1.85.
 11. Article according to claim 9,characterized in that the alkali-resistant glass fibres are present in aproportion of less than 3.5% by weight with respect to the dry matterwithout glass.
 12. Article according to claim 9, characterized in thatthe hydrated matrix contains between 16.5 and 21.5% of water by weightwith respect to the dry matter.
 13. Article according to claim 9,characterized in that the glass fibres have a length/diameter ratio ofbetween approximately 100 and 200:1.
 14. Article according to claim 9,characterized in that the hydrated matrix furthermore contains ultrafineparticles, especially silica fume or metakaolin, in a quantity of 8 to18 parts by weight and preferably 10 parts by weight.
 15. Articleaccording to claim 9, characterized in that the hydrated matrixfurthermore contains an extrusion agent chosen from one of the celluloseethers in a quantity of approximately 1 part by weight.
 16. Articleaccording to claim 9, characterized in that the hydrated matrixfurthermore contains a colorant in a quantity of between 0.5 and 2 partsby weight and preferably 1 part by weight.
 17. Article according toclaim 9, characterized in that the article is shaped and has at leastone face imitating slate.
 18. Article according to claim 17,characterized in that at least the imitation slate face has locally asmooth surface with pores that are at least partially closed. 19.Article according to claim 17, characterized in that the article isintended for use as a building covering element and has a thickness inits central part of less than 1 cm and preferably less than 6 mm. 20.Article according to claim 19, characterized in that along one edge thearticle has catching reliefs projecting downwards and an upper surfacewhich imitates slate and is provided with ribs.
 21. Article according toclaim 15, characterized in that one of the cellulose ethers ismethylhydroxyethylcellulose (MHEC).
 22. Article according to claim 16,characterized in that the colorant is carbon black.